1. Field of the Disclosure
The present disclosure relates to new combretastatins analog compounds and their pharmaceutically acceptable salts; pharmaceutical compositions comprising the new combretastatin analog compounds, either alone or in combination with at least one additional therapeutic agent, with a pharmaceutically acceptable carrier; and uses of the new combretastatin analog compounds, either alone or in combination with at least one additional therapeutic agent, in the treatment of cancer, and in particular cancer presenting as metastatic tumors.
2. Description of Related Art
Inhibition of tubulin polymerization disrupts the formation of tumor vasculature, making the microtubule cytoskeleton an effective target for cancer chemotherapy [1-3]. Combretastatin A4 (CA-4) is the prototype of a large group of vascular disrupting agents that have been designed, synthesized, and tested in various biological models as potential therapeutic candidates for cancer treatment [4, 5]. CA-4 binds to the colchicine binding site of tubulin to block microtubule assembly, causing rapid vascular shutdown in the tumor and cell death [6]. The water-soluble phosphate prodrug form (CA-4P, also known as fosbretabulin) is in phase II/III clinical trials either alone or in combination with traditional chemotherapeutic agents or with radiotherapy [7-10]. Meanwhile, over the past two decades, numerous novel derivatives of CA-4 have been discovered to confer cytotoxic potency and anti-tubulin activity that are comparable to CA-4, significantly expanding the arsenal of vascular disrupting agents that could be further explored for clinical applications.
Modifications made on the two phenyl rings, for example, have led to hundreds of active compounds that possess desirable cytotoxicity while retaining varying degrees of anti-tubulin activities [11]. Most structural variations of the phenyl rings have occurred in the phenyl ring with hydroxyl and methoxy substitutions. These include various substituted phenyl rings [12] and other aromatic rings [13]. A fewer reports have attempted to modify the trimethoxy ring with mixed outcomes. For example, the m-methoxy group has been substituted by a fluoride [14] to yield a similarly potent compound. In another example, when the trimethoxy ring was replaced by a trimethyl ring [15], the cytotoxicity of the compound was significantly reduced but the anti-tubulin activity was largely retained. This suggests that it might be possible to achieve disruption of tumor vasculature with fewer cytotoxic side effects.
Modifications of the double bond have also led to diverse structural variations that remain viable as cytotoxic and anti-tubulin compounds. The olefinic bond is believed to be critical in placing the two phenyl rings at an appropriate distance and giving the molecule the right dihedral angle to maximize the interaction with the target. As such, replacement of the double bond by rings that facilitate a cis-locked configuration has proven to be effective in retaining both cytotoxicity and anti-tubulin activity [16-18]. Indeed, this strategy has led to the discovery of perhaps more active CA-4 analogs than any other types of structural modifications. On the other hand, the observation that two-carbon linkers are the optimal length of the bridge between the two phenyl rings has somewhat limited the effort to explore this strategy of modifications. However, there have been some encouraging exceptions. For example, when the methylene bridge is replaced by a carbonyl group, the resulting analog, phenstatin, actually retained much of the antitubulin activity [19]. Increasing the bridge length to three carbons such as a chalcone-like linker, have been reported to strongly inhibit tubulin polymerization as well as cell survival [20].
Despite the intense interest and the large number of potent derivatives that have been discovered in the therapeutic application of CA-4 and related derivatives targeting the colchicine-binding site of tubulin, none of these inhibitors has reached the clinical stage. Thus there remains an urgent need in developing CA-4 analogs with improved pharmacological properties for eventual acceptance in the clinic.
The present disclosure concerns the design, synthesis, and biological evaluation of a series of pyridine-linked CA-4 analogs (Formula I) in which the distance between the two phenyl rings is configured to be three or four atoms, i.e., meta- or para- to each other. Pyridine has been introduced to replace the cis-double bond between the A ring and B ring [21]. However, it was previously found that while the pyridine A4 derivatives retained some cytotoxicity, the anti-tubulin activities were largely lost. In this disclosure, we show that cytotoxicity and antitubulin activities comparable to CA-4 can be obtained with bridge length fixed at three atoms (including the pyridine nitrogen) if substitutions on one or both of the phenyl rings are optimized. Here we describe the synthesis of 34 pyridine-bridged CA-4 analogs that were tested for their ability to block tubulin polymerization as well as their cytotoxicity to several lines of human cancer cells. Molecular modeling was also performed to better understand the structural requirements for the pyridine-linked CA-4 analogs to retain antimitotic potency.